The present invention relates to a method for fabricating a detection element for detecting light or radiations in which a semiconductor film is used, and a method for fabricating a two-dimensional image detector using the detection element.
Conventionally known as a radiation-sensitive two-dimensional image detector for detecting images by means of radiations has been an apparatus composed of semiconductor sensors (detection elements for detecting radiations) arranged in two dimension that generate charges (electron-hole) upon detection of X-rays. Each semiconductor sensor is provided with an electric switch, and the electric switches are consecutively turned on line by line so that charges in each semiconductor sensor are read out line by line. The Japanese Publication for Laid-Open Patent Application 342098/1994 (Tokukaihei 6-342098 [Publication Date: Dec. 13, 1994]), for example, describes concrete structure and principles of such a radiation-sensitive two-dimensional image detector.
The following description will explain the structure and principles of the foregoing conventional two-dimensional image detector. FIG. 6 is a view schematically illustrating a structure of the foregoing conventional radiation-sensitive two-dimensional image detector. FIG. 7 is a cross-sectional view schematically illustrating a structure of a part of the foregoing radiation-sensitive two-dimensional image detector corresponding to one pixel.
The conventional radiation-sensitive two-dimensional image detector is composed of an active matrix substrate, as well as a photoconductive layer (semiconductor layer) 56, a dielectric layer 57, and an upper electrode 58 that are formed over a substantial entirety of the active matrix substrate. The active matrix substrate includes electrode wires (gate electrodes 52 and data electrodes 53) arranged in an XY lattice form, thin film transistors (hereinafter referred to as TFTs) 54, charge storing capacitors (Cs) 55, and the like provided on a glass substrate 51.
To form the foregoing photoconductive layer 56, a semiconductor material is used in which charges (electron-hole) are generated upon irradiation by radiations such as X-rays. In the foregoing document, as the semiconductor material, amorphus selenium (a-Se) is used that has a high dark resistance and excels in photoconductivity with respect to X-rays. The photoconductive layer 56 is formed to a thickness of 300 xcexcm to 600 xcexcm by the vapor deposition method.
Besides, as the foregoing active matrix substrate, an active matrix substrate formed in a liquid crystal display device fabricating process may be adopted. For example, an active matrix substrate used in an active-matrix-type liquid crystal display device (AMLCD) is arranged to include TFTs made of amorphus silicon (a-Si) or polysilicon (p-Si), electrodes arranged in an XY lattice form, and charge storing capacitors (Cs). Therefore, the AMLCD can be easily used, with slight changes in the design, as the active matrix substrate for use in the radiation-sensitive two-dimensional image detector.
Next, the operational principles of a radiation-sensitive two-dimensional image detector having the foregoing structure are explained below. Upon projection of radiations onto the photoconductive layer 56 composed of an a-Se film or the like, charges (electron-hole) are generated in the photoconductive layer 56. As shown in FIGS. 6 and 7, since the photoconductive layer 56 and the charge storing capacitors (Cs) 55 are connected electrically in series, charges (electron-hole) generated in the photoconductive layer 56 move to the plus electrode side and the minus electrode side upon application of a voltage across the upper electrode 58 and charge storing capacitor electrodes (Cs electrodes) 59. Consequently, charges are stored in the charge storing capacitors (Cs) 55. Each charge storing capacitor (Cs) 55 is equipped with the charge storing capacitor electrode (Cs electrode) 59 and a pixel electrode 60.
By the foregoing operation, the charges stored in the charge storing capacitors (Cs) 55 can be taken to outside through the data electrodes S1, S2, S3, . . . Sn by turning on the TFTs 54 by using input signals to the gate electrodes G1, G2, G3, . . . , Gn. Since electrode wires (gate electrodes 52 and data electrodes 53), TFT 54, and charge storing capacitors (Cs) 55 are all arranged in an XY matrix form, image information on the X lines can be two-dimensionally obtained by line-sequentially scanning signals inputted to the gate electrodes G1, G2, G3, . . . , Gn.
Incidentally, in the case where the photosensitive layer 56 used in the foregoing radiation-sensitive two-dimensional image detector exhibits photoconductivity with respect to not only radiations such as X-rays but also visible light and infrared light, it functions also as a two-dimensional image detector for visible light and infrared light.
In the foregoing conventional two-dimensional image detector, the a-Se film used in the photoconductive layer is directly formed on the active matrix substrate by the vapor deposition method. In the case of such a structure, the following problems arise.
(1) In the case where another semiconductor material is used instead of a-Se to form the photoconductive layer, the semiconductor materials applicable are limited according to the thermal resistivity of the active matrix substrate. For example, a polycrystalline film of CdTe or CdZnTe that expectedly provides improvement of sensitivity with respect to X-rays requires a film forming temperature of not lower than 400xc2x0 C. in the case where it is formed by the MOCVD (metal organic chemical vapor deposition) method, the close-spaced sublimation method, the paste burning method, or the like that is suitable for forming a film on a large-area surface.
On the other hand, a-Si formed in a film form at a temperature of approximately 300xc2x0 C. is generally used in semiconductor layers of TFTs used as switching elements provided on an active matrix substrate. For this reason, a critical temperature of the TFTs regarding heat resistance (hereinafter referred to as heat resistance critical temperature) is approximately 300xc2x0 C. Therefore, it is difficult to form a film of a polycrystalline material such as CdTe or CdZnTe directly on the active matrix substrate.
(2) Generally, the active matrix substrate is fabricated by repeated application of a micromachining process (photolithography) to semiconductors, and naturally the yield thereof decreases as the fabrication process is prolonged. In the case where a charge blocking layer, a photoconductive layer, and an upper electrode are further additionally formed on such an active matrix substrate, the following problem arises: a failure occurring during this addition-type process leads to a drastic fall of the yield in total.
Therefore, applicable to solve the foregoing two problems is a method in which an active matrix substrate and a counter substrate (detection element for detecting light or radiations) including a photoconductive layer are formed separately and independently, and are thereafter assembled so as to be connected with each other by using a conductive connection material. This ensures that the limitation relating to the film formation temperature of the semiconductor layer as the photoconductive layer is relaxed, and that the decrease in the yield can be avoided by combining a non-defective active matrix substrate and a non-defective counter substrate.
Incidentally, by using either anisotropic conductive material or a conductive material arranged in a pattern corresponding to pixels provided separately and independently from one another, a plurality of pixels provided on an active matrix substrate obtain conductivity with a connection surface only in the normal line direction. This enables prevention of electric crosstalk between adjacent pixels within the connection surface.
Appropriately adapted as the anisotropic conductive material is an insulating adhesive (binder resin) in which conductive particles are dispersed, that is, a so-called anisotropic conductive adhesive. Applicable as conductive particles used in the anisotropic conductive adhesive are metal particles such as Ni (nickel) particles, metal particles obtained by plating Ni particles with Au (gold), carbon particles, metal-coated plastic particles such as Au/Ni-plated plastic particles, conductive particle composite plastic obtained by mixing in polyurethane transparent conductive particles such as ITO particles as well as Ni particles, etc. Examples of an adhesive applicable in the foregoing anisotropic conductive adhesive include those of a heat-hardening type, photo-hardening type, and thermoplastic type.
On the other hand, appropriately adapted as the conductive material arranged in the pattern according to pixels separately provided from each other is, for example, (i) a photosensitive resin with a conductivity imparted thereto that can be patterned so as to be provided only on pixel electrodes by the photolithography technique, (ii) a conductive adhesive that can be patterned so as to be provided only on pixel electrodes by the screen printing technique, or (iii) solder bumps.
However, as described above, problems described below arise in the case where an active matrix substrate and a counter substrate including a semiconductor layer with photoconductivity are separately formed and are then assembled so as to be connected with each other by using a conductive connection material, to form a two-dimensional image detector.
FIG. 8 is a cross-sectional view schematically illustrating a structure of a part of a two-dimensional image detector corresponding to one pixel. The two-dimensional image detector is formed in the following manner: an active matrix substrate 61 and a counter substrate (detection element for detecting light or radiations) including a semiconductor layer 66 having photoconductivity, having been separately formed, are assembled with a conductive adhesive 69 arranged in a pattern according to pixels that are separately provided from each other.
In the foregoing two-dimensional image detector, the counter substrate 62 including the semiconductor layer 66 is formed by laminating an upper electrode 64, a first charge blocking layer 65 as required, a semiconductor layer 66 having photoconductivity, a second charge blocking layer 67 as required, and a connection electrode 68 in this order on over a substantial entirety of a surface of a support substrate 63.
Since the first and second charge blocking layers 65 and 67 among them are intended to reduce dark current (current leakage) of the semiconductor layer 66, they may be provided as required. The foregoing connection electrode 68 is aimed for collecting charges at each pixel.
In the case where the aforementioned polycrystalline semiconductor material such as CdTe or CdZnTe is used for forming the foregoing semiconductor layer 66, the semiconductor layer 66 need be formed to a film thickness (layer thickness) of several hundreds of xcexcm, in consideration of the efficiency in absorbing X-rays. However, it has been discovered that, in the case where polycrystalline CdTe is formed to a thickness of several hundreds of xcexcm, random projections and recesses are produced on the surface. This phenomenon is observed also in the case where CdTe or CdZnTe is provided in a film form by any method such as the MOCVD method, the close-spaced sublimation method, or the paste burning method.
The following three problems (a) through (c) arise in the case where projections and recesses are produced on the surface of the semiconductor layer 66 as described above.
(a) A desirable junction state cannot be obtained at the interface between the semiconductor layer 66 and the second charge blocking layer 67, resulting in that a sufficient charge blocking effect cannot be obtained. More specifically, when to obtain a charge blocking effect is attempted by junction of the semiconductor layer 66 to the second charge blocking layer 67 (by, for example, the PIN junction, or the Schottky junction, the MIS (metal insulator semiconductor) junction, or the hetero junction), a problem that the second charge blocking layer 67 is not formed in a good state arises, and a sufficient charge blocking effect cannot be achieved. The reason is as follows: since the second charge blocking layer 67 is formed on the surface of the semiconductor layer 66 having projections and recesses, the second charge blocking layer 67 is not adequately formed on, for example, top parts of projections of the semiconductor layer 66, and leakage occurs there.
(b) Upon assembly of the active matrix substrate 61 and the counter substrate 62, projections and recesses are also produced on the connection electrode 68 on the surface of the counter substrate 62 according to the projections and recesses on the surface of the semiconductor layer 66, thereby causing defects in contact between the connection electrode 68 and the conductive connection material 69.
For example, in the case where an anisotropic conductive adhesive is used as the conductive connection material 69, conductive particles are buried in recesses of the connection electrode 68, and hence, cannot be sufficiently flatly dispersed. This causes connection defects to likely occur and reliability to deteriorate.
Furthermore, in the case where a conductive material (for example, photoconductive resin) patterned according to pixels is used as the conductive connection material 69, bubbles tend to be caught between the connection electrode 68 and the conductive connection material 69, thereby likely leading to deterioration of reliability. Furthermore, the conductive connection material 69 need be patterned according to pixels independent and separate from one another, but the pattern form of the conductive connection material 69 tends to vary due to influences of the projections and recesses on the surface of the connection electrode 68, thereby resulting in that portions of the conductive connection material 69 corresponding to adjacent pixels are likely brought into contact with each other.
(c) In the case where the connection electrode 68 that collects charges is provided on the second charge blocking layer 67 on the semiconductor layer 66, or on the semiconductor layer 66 (without the second charge blocking layer 67), it is difficult to conduct micromachining of the connection electrode 68 if the surface of the semiconductor layer 66 has projections and recesses.
Furthermore, a polycrystalline film of CdTe or CdZnTe can be also used in a device other than the two-dimensional image detector, for instance, a solar battery that is a sort of a detection element for detecting light. An example of a CdS/CdTe thin film solar battery is reported in Matsushita Technical Journal, Vol.44, No.4, pp.477-488 (1998).
To briefly explain a fabrication process of the foregoing CdS/CdTe thin film solar battery, a CdTe polycrystalline thin film whose crystalline particle diameter is about 3 xcexcm is formed about 6 xcexcm in thickness. Furthermore, after the crystalline particle diameter of CdTe is caused to grow by thermal treatment to about 5 xcexcm, a conductive paste of carbon or the like is applied by printing on the CdTe film to form an upper electrode. Thus, a solar battery is completed.
Here, projections and recesses that have a difference in height of about 3 xcexcm (xc2x11.5 xcexcm) are likely produced on the surface of the CdTe film, reflecting shapes of the crystalline particles with the diameter of about 5 xcexcm. Therefore, in printing the conductive paste thereon, failures in printing tend to occur due to the projections and recesses on the surface.
An object of the present invention is to provide a method for fabricating a detection element for detecting light or radiations in which effective improvement of a charge blocking effect and improvement of reliability can be achieved by flattening a surface of a semiconductor film having projections and recesses, and further, to provide a method for fabricating a two-dimensional image detector in which the foregoing detection element for detecting light or radiations is applied.
To achieve the foregoing object, a method for fabricating a detection element in accordance with the present invention is characterized by including the steps of (i) forming a semiconductor film having photoconductivity on a substrate, (ii) flattening a surface of the semiconductor film, and (iii) forming at least either a charge blocking layer or an electrode layer on the semiconductor film.
Furthermore, preferably used as the foregoing semiconductor film is a semiconductor film that exhibits photoconductivity with respect to at least either visual light or infrared light projected thereto, or alternatively, a semiconductor film exhibiting photoconductivity with respect to radiations projected thereto.
According to the foregoing method, the semiconductor film is formed on the substrate, a surface of the semiconductor film is then flattened, and thereafter, at least either a charge blocking layer or an electrode layer is formed on the semiconductor film. Therefore, when at least either a charge blocking layer or an electrode layer is formed on the semiconductor film, the semiconductor film has a surface that is flat, having substantially no projections or recesses.
This enables to obtain an ideal connection state at an interface between the semiconductor film and the charge blocking layer in the case where the charge blocking layer is formed on the semiconductor film. Therefore, this enables to suppress occurrence of leakage at the interface between the semiconductor film and the charge blocking layer, thereby ensuring an effective charge blocking effect. Further, in the case where the charge blocking layer or the electrode layer is formed on the semiconductor film, the micromachining process can be easily applied for forming the charge blocking layer or the electrode layer since the surface of the semiconductor film is flattened.
Furthermore, the method for fabricating a detection element in accordance with the present invention is preferably arranged so that in the flattening step, the surface of the semiconductor film is sprayed with ceramic particles.
Usually, the method of spraying a substrate as a work with ceramic particles is applied for the purpose of arbitrarily producing random projections and recesses on a surface of the substrate. Therefore, by the foregoing method in which the semiconductor film having projections and recesses on its surface is sprayed with ceramic particles, conversely the flatness of the semiconductor film surface can be improved. Since such a ceramic particle spraying method is very simple, it is possible to realize flatness of the semiconductor film surface by means of a simple device.
Thus, it is possible to easily make a semiconductor film formed on a large-area substrate possess a flat surface.
Incidentally, the xe2x80x9cceramic particlesxe2x80x9d described herein generally refer to particles made of a solid substance other than a metal.
A method for fabricating a two-dimensional image detector in accordance with the present invention is a method for fabrication a two-dimensional image detector composed of (1) an active matrix substrate having (i) a pixel array layer including electrode wires arranged in a lattice form, (ii) switching elements provided at lattice points of the electrode wires, and (iii) charge storing capacitors including pixel electrodes connected with the electrode wires through the switching elements, (2) a counter substrate having a support substrate on which a first electrode layer and a semiconductor film having photoconductivity are provided in the stated order, and (3) a conductive connection material electrically connecting the active matrix substrate and the counter substrate in a state in which the pixel array layer of the active matrix substrate and the semiconductor film of the counter substrate are vis-a-vis each other, and the method is characterized by including the steps of (a) forming the first electrode layer on the support substrate, (b) forming the semiconductor layer on the first electrode layer, (c) flattening a surface of the semiconductor film, and (d) forming either a charge blocking layer or a second electrode layer on the semiconductor film.
According to the foregoing method, an active matrix substrate provided with a pixel array layer and a counter substrate having a semiconductor film with photoconductivity are separately fabricated, and thereafter, these substrates are electrically connected with a conductive connection material. On the other hand, the counter substrate is fabricated as follows: a first electrode layer and a semiconductor film are formed in this order on a support substrate, a surface of the semiconductor film is then flattened, and thereafter, either a charge blocking layer or a second electrode layer are formed thereon. Therefore, when the charge blocking layer or the second electrode layer is formed on the semiconductor film, the semiconductor film surface is flat, having substantially no projections or recesses.
This enables to obtain an ideal connection state at an interface between the semiconductor film and the charge blocking layer in the case where the charge blocking layer is formed on the semiconductor film. Therefore, this enables suppression of occurrence of leakage at the interface between the semiconductor film and the charge blocking layer, thereby ensuring an effective charge blocking effect. Further, in the case where the charge blocking layer or the second electrode layer functioning to collect charges is formed on the semiconductor film, the micromachining process can be easily applied for forming the charge blocking layer or the second electrode layer since the surface of the semiconductor film is flattened.
Furthermore, the method for fabricating a two-dimensional image detector in accordance with the present invention is preferably arranged so that in the flattening step, the surface of the semiconductor film is sprayed with ceramic particles.
Usually, the method of spraying a substrate as a work with ceramic particles is applied for the purpose of arbitrarily producing random projections and recesses on a surface of the substrate. Therefore, by the foregoing method in which the semiconductor film having projections and recesses on its surface is sprayed with ceramic particles, conversely the flatness of the semiconductor film surface can be improved. Since such a ceramic particle spraying method is very simple, it is possible to realize flatness of the semiconductor film surface composing the two-dimensional image detector by means of a simple device.
Thus, it is possible to easily make a semiconductor film in the two-dimensional image detector possess a flat surface even in the case where it is formed on a large-area support substrate.
Incidentally, the xe2x80x9cceramic particlesxe2x80x9d described herein generally refer to particles made of a solid substance other than a metal.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.